Google Chrome: People who spend a lot of time on Wikipedia may find this extension particularly valuable, even if it is somewhat limited. With WikiPreview installed, any time you hover your mouse over a link to another Wikipedia article while on the site, a small snippet pops up to show you what that article's about.

Unfortunately, it only works while you're actually on Wikipedia, and it won't show a snippet of a subsection should you hover over a "#" link (yet). It may be limited, but it does make browsing and researching on the site a much more relaxed experience.